This application claims the benefit of a Japanese Patent Application No.2002-056846 filed Mar. 4, 2002, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to optical communication systems employing Raman amplification, and more particularly to an optical communication system which employs the Raman amplification with a satisfactory nonlinear characteristic and is capable of making a communication regardless of a location of a disconnection in an optical fiber which forms a trunk transmission line.
2. Description of the Related Art
After the optical communication system was developed in the 1950's, an optical regenerative repeater used when making the optical communication over a long distance had three basic functions, namely, reshaping, retiming and regenerating. But in order to realize the three basic functions, there were problems in that the structure of the optical regenerative repeater became complex, a large number of adjustments was required in the optical regenerative repeater, and the cost reduction of the optical regenerative repeater became difficult.
In addition, even though the optical signal is normally unaffected by the electromagnetic waves, the optical regenerative repeater must once convert the optical signal into an electrical signal, and regenerate the waveform before converting the electrical signal back into the optical signal, when performing the repeater function. As a result, considerations had to be made in the electrical design and the packaging design in order to prevent deterioration of a bit error rate caused by the repeater function, thereby making it more difficult to reduce the cost of the optical regenerative repeater.
In the 1980's, an optical fiber amplifier was developed. The optical fiber amplifier uses an optical fiber having a core added with ions of a rare earth element, such as erbium ions. The optical fiber amplifier is not only used in place of the optical regenerative repeater, but is also used as an output amplifier of a terminal station in various optical communication systems.
In a case where an optical fiber amplifier, using an erbium-added optical fiber which has the core added with the erbium ions, is used as the repeater of the optical communication system, it is possible to greatly reduce the number of parts within the repeater and to greatly reduce the number of adjustments which are required in the repeater. Consequently, it is possible in this case to reduce the cost and improve the reliability of the optical communication system.
At the same time, the transmission capacity required of the optical communication system has continued to increase, and there are now demands to further increase the transmission speed. The optical fiber is originally subjected to a waveform deterioration factor, that is, wavelength dispersion (often simply referred to as “dispersion”) which corresponds to group delay distortion which is generated when transmitting electrical signals. For this reason, there was a problem in that the transmission quality of the optical fiber deteriorates more as the transmission rate becomes higher.
A system has been proposed which combines an optical fiber having a positive dispersion slope (hereinafter simply referred to as a “positive dispersion fiber”) and an optical fiber having a negative dispersion slope (hereinafter simply referred to as a “negative dispersion fiber”) in order to form a trunk transmission line and compensates for the dispersion. The positive dispersion fiber has a large core diameter, and deterioration of the transmission quality caused by nonlinear distortion is small. The negative dispersion fiber has a small core diameter in order to realize the negative dispersion slope, and the deterioration of the transmission quality caused by the nonlinear distortion is generally large.
However, by arranging the positive dispersion fiber which is less affected by the nonlinear distortion in the first half of the trunk transmission line and arranging the negative dispersion fiber in the second half of the trunk transmission line, the level of the optical signal input to the negative dispersion fiber becomes low due to attenuation of the optical signal while being transmitted through the positive dispersion fiber. Hence, it is possible to suppress the generation of the nonlinear distortion in the negative dispersion fiber. Therefore, a long-distance optical communication system having satisfactory characteristics can be formed by the repeater which is made up of the optical fiber amplifier using the erbium-added optical fiber, and the optical transmission line which is made up of the combination of the positive dispersion fiber and the negative dispersion fiber.
Due to the recent globalization and rapidly increasing used of the Internet, the demands for increased transmission capacity with respect to the optical communication system continue to increase. In order to cope with such demands, a wavelength division multiplexing (WDM) technique has been proposed. According to the WDM technique, a plurality of optical signals obtained by respectively modulating lights having a plurality of wavelengths by different electrical signals are multiplexed and transmitted through a single optical fiber. Active research on the WDM technique itself was made in the 1950's, but the application to a high-speed large-capacity optical communication system has been made only recently. The broadening of the band and the increasing of the capacity in the optical communication system have been made in this manner.
A problem was encountered in the transmission band of the repeater which is made up of the optical fiber amplifier using the erbium-added optical fiber due to the broadening of the band of the optical communication system. However, by adding a gain equalizer or a loss equalizer having desired characteristics with respect to the optical signal, it was possible to compensate for the wavelength characteristic of the gain of the optical fiber amplifier using the erbium-added optical fiber, and to widen the wavelength region in which the gain of the optical fiber amplifier using the erbium-added optical fiber is constant. But with respect to the demands to further broaden the band of the optical communication system, there are views that virtually the limit has been reached in coping with such demands.
On the other hand, a Raman amplifier which uses the stimulated Raman scattering (SRS) phenomenon and has a broad transmission band has been proposed.
FIG. 1 is a diagram for explaining an example of a conventional optical communication system employing the Raman amplification. It is assumed for the sake of convenience that the conventional optical communication system shown in FIG. 1 is applied to an optical submarine communication system. In FIG. 1, only an up-line or down-line is shown.
Terminal repeating installations 101 and 101a shown in FIG. 1 are respectively provided within a terminal station which is set up at a boundary of a ground transmission line and a submarine transmission line. Each of the terminal repeating installations 101 and 101a includes a transmission unit for a main signal, a transmission unit for a monitor signal, and a power supply unit for submarine repeaters.
One span of trunk transmission line is formed by a positive dispersion fiber 103 and a negative dispersion fiber 106, and one span of trunk transmission line is formed by a positive dispersion fiber 103a and a negative dispersion fiber 106a. 
A repeater is provided at a boundary between two trunk transmission lines which are respectively made up of the positive dispersion fiber and the negative dispersion fiber. Only a portion of the repeater is shown in FIG. 1. One repeater on the left side in FIG. 1 includes a light source 104a for Raman pumping (hereinafter referred to as a Raman pump light source 104a) for generating a Raman pump light, and a coupler 105a for coupling the Raman pump light emitted from the Raman pump light source 104a to the negative dispersion fiber 106. Another repeater on the right side in FIG. 1 includes a Raman pump light source 104c for generating a Raman pump light, and a coupler 105b for coupling the Raman pump light emitted from the Raman pump light source 104c to the negative dispersion fiber 106a. 
An optical signal which is transmitted at a regular level from the terminal repeating installation 101 attenuates within the positive dispersion fiber 103, but is amplified by an energy of the Raman pump light while propagating within the negative dispersion fiber 106 which is pumped by the Raman pump light. Hence, the optical signal has a sufficiently high level at an output end of the negative dispersion fiber 106. The level of the optical signal at the output end of the negative dispersion fiber 106 is determined by taking into consideration the attenuation of the optical signal due to losses in a branching filter and a gain equalizer which are not show in FIG. 1 but are provided in the repeater. Hence, the level of the Raman pump light and the length of the negative dispersion fiber 106 are appropriately determined in order to determine the level of the optical signal at the output end of the negative dispersion fiber 106.
FIG. 2 is a diagram showing an optical signal level transition in one span of the conventional optical communication system employing the Raman amplification. FIG. 2 shows a case where the positive dispersion fiber has a length of 33 km, and the negative dispersion fiber has a length of 17 km.
In FIG. 2, the ordinate indicates the optical signal power in dBm, and the abscissa indicates a transmission distance in km. The unit dBm is a unit of power with reference to 1 mW, which is obtained from 10log10(P mW/1 mW), where P denotes the optical signal power. In this case, a loss of approximately 6 dB occurs in the positive dispersion fiber. Hence, the negative dispersion fiber needs to compensate for this loss, and in an ideal case where there is no loss in the repeater, the negative dispersion fiber only needs to amplify by approximately 6 dB. However, the loss does occur in the repeater as described above, and the amplification must be made in the negative dispersion fiber by taking into consideration this loss in the repeater. In this example, it is assumed for the sake of convenience that the loss in the repeater is approximately 7 dB, and thus, the negative dispersion fiber must make an amplification by approximately 13 dB.
Thereafter, the optical signal is transmitted similarly, and finally reaches the terminal repeating installation 101a. 
As described above, the negative dispersion fiber has a small core diameter, thereby making it difficult to suppress the deterioration of the transmission quality caused by the nonlinear distortion. On the other hand, the repeater positively utilizes the nonlinear distortion caused by the small core diameter, so as to make an amplification by stimulated Raman scattering (SRS).
FIGS. 3A and 3B are diagrams for explaining a first problem of the conventional optical communication system employing the Raman amplification. FIGS. 3A and 3B show cases to explain that, when a disconnection occurs in an optical cable, it may not be possible to transmit information indicating the disconnection from the repeater to the terminal repeating installation, depending on the location of the disconnection. In FIGS. 3A and 3B, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted.
FIG. 3A shows a case where the disconnection occurs in the optical cable at a location distant from the repeater. More particularly, the disconnection is located in a vicinity of a boundary between the negative dispersion fiber 106 and the positive dispersion fiber 103 in FIG. 3A.
In this case, the Raman pump light is coupled by the negative dispersion fiber 106 and can propagate in a direction opposite to a transmitting direction of the optical signal. Since the optical cable has the disconnection and the optical signal cannot propagate through the optical cable, the Raman pump light cannot amplify the optical signal. However, spontaneous emission light is generated as a result of the Raman pump light applying molecular vibration energy to the material forming the optical fiber of the optical cable. Moreover, the power of the spontaneous emission light reaching the repeater is relatively large because the optical signal is not amplified. Accordingly, if the Raman pump light is modulated by a specific electrical signal in the repeater, the spontaneous emission light can be modulated by the specific electrical signal, and the modulated spontaneous emission light can reach the terminal repeating installation 101a while being amplified in the subsequent span. In other words, it is possible in this case to transmit information indicating the disconnection of the optical cable to a maintenance person at the terminal repeating installation 101a. 
On the other hand, FIG. 3B shows a case where the disconnection occurs in the optical cable at a location close to the repeater. More particularly, the disconnection is located in a vicinity of a boundary between the negative dispersion fiber 106 and the repeater in FIG. 3B.
In this case, the Raman pump light is coupled by the negative dispersion fiber 106 but can propagate for only a short distance in a direction opposite to the transmitting direction of the optical signal. Hence, the power of the spontaneous emission light which is generated is small. For this reason, even when the Raman pump light is modulated by the specific electrical signal in the repeater, it is not possible to obtain a modulated spontaneous emission light which is modulated by the specific electrical signal and has a sufficiently high power. In this case, the information indicating the disconnection of the optical cable cannot be transmitted to the maintenance person at the terminal repeating installation 101a. 
In addition, although it is possible to transmit the disconnection information indicating the disconnection at the location distant from the repeater and not the disconnection information indicating the disconnection at the location close to the repeater, it is impossible to accurately distinguish the distant and close locations of the disconnections relative to the repeater. In other words, it is impossible to accurately distinguish the case where the disconnection information can be transmitted to the maintenance person at the terminal repeating installation 101a and the case where the disconnection cannot be transmitted to the maintenance person at the terminal repeating installation 101a. 
FIG. 4 is a diagram showing a nonlinear characteristic in one span of the conventional optical communication system employing the Raman amplification. In addition, FIG. 5A is a diagram showing a structure of one span in the conventional optical communication system, which takes a trunk transmission line as an example. FIG. 5B is a diagram showing a structure of one span in an optical communication system according to the present invention, and will be described later.
The structure shown in FIG. 5A includes repeaters 107 and 107a, a positive dispersion fiber 103, and a negative dispersion fiber 106. The repeaters 107 and 107a are indicated by lines, because it is difficult to accurately illustrate the length of the trunk transmission line if the repeater which is small within the trunk transmission line is illustrated by a block.
In FIG. 4, the ordinate indicates a nonlinear amount of the optical signal, and the abscissa indicates a distance from the repeater 107. The nonlinear amount is proportional to [(power of optical signal)×(nonlinear refractive index)/(nonlinear effective cross sectional area of core)]. Accordingly, the nonlinear amount takes extremely small values in the positive dispersion fiber 103 for a distance of up to 33 km from the repeater 107.
The power of the optical signal remains unchanged at a node between the positive dispersion fiber 103 and the negative dispersion fiber 106, located 33 km from the repeater 107. However, since the cross sectional area of the core becomes discontinuous at this node, the nonlinear amount makes a step increase.
Thereafter, as the optical signal propagates from the node located 33 km from the repeater 107 towards the repeater 107a located 50 km from the repeater 107, the amplification by the Raman pump light increases, thereby increasing the power of the optical signal. For this reason, the nonlinear distortion generated within the negative dispersion fiber 106 gradually increases, and becomes a maximum at a node between the negative dispersion fiber 106 and the repeater 107a. Moreover, because the optical signal must be amplified to a level which takes into consideration the loss of the repeater 107a as described above, the nonlinear distortion undergoes a corresponding increase.
The optical communication system employing the Raman amplification does not carry out a regenerative repeater function. Consequently, the nonlinear distortion is accumulated as the span of the repeater increases, to thereby deteriorate the transmission quality.